Solo player who doesn't do powerplay... 
Scenario 1. Players who gave up on powerplay / the game because they couldn't shoot other players come back, solo/ group players see the 'error of their ways' and flock to open. Open thrives, so does powerplay.
Scenario 2. Players who enjoy to powerplay in solo / group stop playing powerplay / the game. Powerplay is even less used and open is much the same.
Scenario 3. A balance of the above with some players coming back, and some stopping. Not much changes with regards to powerplay. The day that it becomes open only, the forum is flooded with threads about how groups blockaded systems / stations, destroyed a few of the enemy, but are furious because clearly some people that they didn't see (instancing, platforms, time zones..) obviously got through and they didn't 'win'.
And the game will still be the same, blind RNG, USS hunting, enemies announced as incoming just as you dock and factions sending you on missions and then putting a bounty on your head / ship when you complete it for them...
Scenario 1. Players who gave up on powerplay / the game because they couldn't shoot other players come back, solo/ group players see the 'error of their ways' and flock to open. Open thrives, so does powerplay.
Scenario 2. Players who enjoy to powerplay in solo / group stop playing powerplay / the game. Powerplay is even less used and open is much the same.
Scenario 3. A balance of the above with some players coming back, and some stopping. Not much changes with regards to powerplay. The day that it becomes open only, the forum is flooded with threads about how groups blockaded systems / stations, destroyed a few of the enemy, but are furious because clearly some people that they didn't see (instancing, platforms, time zones..) obviously got through and they didn't 'win'.
And the game will still be the same, blind RNG, USS hunting, enemies announced as incoming just as you dock and factions sending you on missions and then putting a bounty on your head / ship when you complete it for them...